Adipic acid crystallization



Nov. 12, 1957 w. B. CLARK ET AL 2,813,122

ADIPIC ACID CRYSTALLIZATION Filed Aug. 27, 1953 ANALYTICAL BALANCESATURATOR f FLUIDIZATION CHAM BER SUPERSATURATOR ROTAMETER FILTER PUMPINVENTORS WILLIAM B.CLARK ROBERT E. GEE

ATTORNEY nited tats ADIPIC ACID CRYSTALLIZATIQN Application August 27,1953, Serial No. 37 6,849

9 Claims. (Cl. 260-537) This invention relates to a process for thecrystallization of adipic acid and more particularly to a process ofcarrying out that crystallization of adipic acid at a high rate from theproducts obtained by oxidation of cyclic hydrocarbons or mixtures of thecyclic hydrocarbons and other hydrocarbons. This application is acontinuationin-part of application S. N. 278,066, filed March 22, 1952,now abandoned.

The petitioners process relates more particularly to the economicrecovery of adipic acid in substantially pure form by crystallizationfrom aqueous solutions of the products obtained from these oxidations.Adipic acid is produced commercially from cyclohexane, for instance, bysuccessive oxidations. The cyclohexane may be treated with air accordingto the Loder U. S. Patent 2,223,493, issued December 3, 1940, so as toconvert a portion of the cyclohexane to partially oxidizedintermediates. The partially oxidized intermediates are usually firstseparated from unoxidized hydrocarbon and may then, without furtherpurification, be oxidized to adipic acid in high yield by treatment withnitric acid as in the Hamblet et al. U. S. Patent 2,439,513, issuedApril 13, 1948. Alternatively, the partially :oxidized intermediates maybe fractionated and selected portions subjected to oxidation by nitricacid or air to form the adipic acid.

All of these large scale commercial processes for the manufacture ofadipic acid by oxidation of cyclic hydrocarbons have certain features incommon. In each case, irrespective of the purification steps enroute,the crude reaction product from the final air or nitric acid oxidationcontains by-product organic acids; such monobasic acids as acetic,valeric, caproic, butyric, cyclohexanecarboxylic, isovaleric, andpropionic, and such polybasic acids as succinic, glutaric and oxalic. Ineach case the crude product of the oxidation comprises a solution of theadipic and lay-product acids in some solvent, and recovery of the adipicacid is accomplished by crystallization and subsequent recrystallizationfrom an aqueous medium. In all of these crystallizations various motherliquor and filter cake wash streams containing adipic and by-productacids are recycled to the crystallization steps. Heretofore, it has beencustomary to restrictthe portion and composition of the recycle streamsto maintain in the crystallization environment concentrations ofby-product acids as low as possible in an effort toobtain a finalproduct of high purity.

An object of the present invention is to provide an improved process forthe crystallization of adipic acid. A further object is to provide acommercially attractive process for crystallizing adipic acidfromaqueous solutions containing other acids. Yet another object is tocrystallize adipic acid in the form of large crystals by providingconditions for increasing the rate of crystal growth and minimizing therate of nucleation of adipic acid from the products of air and/or nitricacid oxidation of cyclic hydrocarbons and mixtures of cyclic hydroatcntice carbons with other hydrocarbons. Other objects and advantages of theinvention will appear hereinafter.

in the various crystallizations of adipic acid from aqueous media thereis a pronounced tendency to produce small crystals. When saturatedsolutions are cooled appreciably, the rate of formation of new crystalnuclei is high in relation to the rate of growth of existing crystals."the high rate of nucleation is even more apparent when nitric acid ispresent in the aqueous medium. As a result, the adipic acid isfrequently obtained in the form of a milky dispersion. The capacity offiltration equipment separating the adipic acid from mother liquor isgreatly reduced when the crystal size is small. In addition, the smallcrystals retain greater portions of mother liquor, washing proceduresbecome less effective, and the purity of the final product is impaired.

In accord with the invention it has been found that the presence ofother organic acids in an aqueous adipic acid crystallizationenvironment reduces the rate of formation of new crystal nuclei. Theseorganic acids are in corporated in concentration below their solubilitylimit. As a result, adipic acid can be separated in the form of largecrystals from solutions that would otherwise give only finely dividedparticles. Adipic acid of high purity can thus be obtained because ofimproved filtration and washing characteristics.

Both the monobasic and dibasic by-productorganic acids have been usedwith success to suppress nucleation. The monobasic acids are in generalmore eifective than the dibasic acids, and smaller amounts are required.However, the monobasic acids are adsorbed in trace quantities on thesurface of the adipic acid crystals, thus requiring for some end usesfurther purification such as prolonged high temperature drying. Thedibasic acids show less tendency for adsorption and the normal waterwashing techniques produce adipic acid crystals of high purity.

in the commercial processes mentioned above, there are several ways ofintroducing the organicacids in sufficient quantity to suppressnucleation to the desired degree. They can, of course, be introduced assuch into the crystallizer feed. As an alternative, hydrocarbons thatproduce the desired acids in high yield can be introduced into theinitial air oxidation step. However, since eifective organic acids areproduced even from cyclohexane in appreciable quantities, we prefer tobuild-up the proper concentration of by-product acids in thecrystallization environment by recycling to the process, streams inwhich these byproducts are concentrated.

While the by-product organic acids are remarkably effective insuppressing the nucleation of adipic acid crystals, the rate of crystalgrowth is also decreased appreciably by their presence. Thus, undercertain conditions it is quite possible to cool such solutions as muchas 10 C. below the true adipic acid crystallizing point for severalhours with little apparent effect. No nucleation is evident andsupersaturation is released only very slowly on existing crystalsurfaces. It is obvious that additional conditions must be stipulatedfor a commercially useful crystallization process.

In further accord with this invention, it has been found highlyimportant to conduct adipic acid crystallizations at elevatedtemperature As a result of extensive research it has been determinedthat a large positive temperature co-eficient exists for the rate ofcrystal growth of adipic acid from aqueous solutions. Comparing thecrystallization of solutions of comparable supersaturation (weight ofcrystallizable adipic acid/volume of solution), quantitative resultsshow that increasing the temperature from 25 C. to 47 C. increases thecrystal growth rate as much as ten-fold. Practical process considerations of handling the saturated solutions containing high concentrationsof adipic acid limit the upper crystallizer temperature to about 70 C.

In the commercial processes we have discussed previously, highercrystallization temperatures can be used conveniently withoutsacrificing recovery of adipic acid due to greater solubility in thehigher temperature mother liquor. After crystallization and filtrationsteps, the major portion of mother liquor is recycled to the process,taking only sufiicient purge to maintain the by-product dibasic acidconcentration below the point where they would crystallize out with theadipic acid. When the temperature of crystallization is raised, higherconcentrations of by-product dibasic acids can be tolerated without suchco-crystallization. The portion of mother liquor that is purged can thusbe reduced, thereby compensating for the greater solubility of adipicacid at the higher temperature. In all cases the purge stream isprocessed further to recover additional adipic acid.

It has been found that excellent crystallizer operation, e. g., highcrystal growth rate, and low crystal formation (nucleation) rate or highsolution stability, occur at temperatures between 40 C. to 70 C. with aslittle as 0.1% up to 0.5% or more, based on the total weight of thesolution, of .a by-product monobasic acid such as caproic acid, whencrystallizing from ordinary aqueous solutions with somewhat higherpercentages, about 3%, for the monobasic. acid having less than 8 carbonatoms. Approxi- .mately l% of a by-product dibasic acid, based on thetotal weight of the solution, such as succinic acid gives comparableresults. However, when nitric acid is a component of the crystallizationenvironment, the rate of nucleation is increased and larger amounts ofby-product acid are required to stabilize the supercooled solution. Forinstance, an aqueous adipic acid solution containing 40% nitric acidrequires about 8% succinic acid for good crystallizer performance at 40C. Up to 18% of the dibasic acids may, however, be used, thesupersaturation of adipic acid in nitric acid solutions of from 25% to45% being between 2 C. and about C. for high solution stability attemperatures between 40 C. and 70 C.

The invention will be more readily understood and the terminologyemployed more clearly appreciated by reference to the drawing whichdiagrammatically illustrates the method used for determining the optimumconditions for optimum rates of growth; Although the crystallization wasstudied in a continuous system, the principles are equally applicable tobatch crystallizations.

In vessel 1 a solution is saturated with adipic acid. From the saturatorvessel 1 the saturated solution is forced by pump 2 through filter 3 androtameter 4 into the supersaturator 5 wherein the temperature of thesolution is lowered to give the desired degree of supersaturation. Fromvessel 5 the supersaturated solution flows through the fiuidizationchamber 6 containing in the bottom portion thereof a suitable porousseed basket 7 in which the crystals are collected. From the top offiuidization chamber 6 the mother liquor is returned to the saturator 1.The yield of crystals collected in seed basket 7 can be weighed fromtime to time to determine the rate of crystallization.

In determining the rate of crystal growth a fluid concentrate wasprepared by saturating water with 100- mesh (U. S. standard) adipic acidat carefully controlled temperature for from 1 to 1 /2 hours. Thisconcentrate was placed in the saturator 1 while a carefully weighedquantity of 10-20 mesh adipic acid seed crystals, with known surfacearea characteristics, were placed in the seed basket 7 and the loadedbasket inserted in the crystallization chamber 6. The saturated solutionwas then forced by pump 2 through filter 3, rotameter 4 andsupersaturator 5 in which the temperature of the saturated liquor waslowered to the desired supersaturation temperature. The flow rate,liquor temperature in the fiuidization chamber and liquor temperature inthe saturator were taken and after prescribed periods of operation theseed basket of crystals was weighed so that a semi-continuous record ofweight gain versus time was obtained and the crystallization ratecalculated.

To demonstrate the effect of by-product acids on suppressing thenucleation rate of supersaturated aqueous adipic acid solutions, arepresentative group of runs made in the experimental laboratorycrystallizer described above are tabulated in Table I. Inspection ofthese data indicates that bulk nucleation occurs abruptly above certaincritical values of the supersaturation, and that these criticalsupersaturation values are influenced markedly by by-product monobasicor dibasic acid concentration. For example, increasing the absolutesupersaturation from 0.349 lbs./ft. (run 1A) to 0.437 lbs./ft. (run 1B)resulted in bulk nucleation, causing the laboratory unit to plug andbecome inoperable. In runs 5A and 5B, increasing supersaturation from0.365 lbs/ft. to 0.391 lbs/ft. caused uncontrolled nucleation to occur.Runs 6, 7, 8, and 9 also illustrate the transition beyond the criticalsupersaturation point where nucleation occurs. The effect of by-productorganic acids on the solution stability can be seen by comparing runs 1and 2, or runs 8, 9 and 10. In the latter three runs, uncontrollednucleation did not occur until supersaturations of 0.966, 1.080, andover 1.24 lbs/ft. were reached in solutions containing 2, 8 and 12%by-product dibasic acids, respectively. The efiect of smallconcentrations of monobasic acids on stability can be seen by comparingruns 2 and 6, where nucleation occurred without added caproic acid atless than 0.239 lbs/ft. supersaturation (run 2), but with 0.037% caproicacid added, supersaturations of 0.358 lbs/ft. could be tolerated withoutinstability.

It is further shown from Table I that increasing amounts of by-productorganic acids are necessary to stabilize the solutions when the nitricacid strength is increased. In runs 1, 2, and 3 the nitric acid strengthwas 0, 10, and 40% and the solutions became unstable at supersaturationsof 0.349, 0.239, and 0.113 lbs./ft. respectively. Runs 5, 6 and 7 alsoillustrate this point.

TABLE I Adipic acid crystallization from aqueous solutions (38-42" C.)

supersat- Run No. uration Conditions Remarks (lbs/it. solution) 1A 0.3490% HNO3, 0% By- Stable operation of Product Acids. unit. 1B 0.437 d0Bulknucleation-Unit plugged. 2 0.239 10% HNO:, 0% By- No meaningful rateProduct Acids. data could be obtained. Unstable. 3 0.113 40% HNO;, 0%By- Copius bulk nuclea- Produet Acids. tion. 4 0.590 0% HNOa, 5% Suc-Good run.

0111 Acid. 5A 0.365 5% ENG, 0.037% Ca- Do.

proic Acid. 0.391 o Nucleation occurred. 0.358 10% HNOa, 0.037% Goodrun.

Caproic Acid. 6B 0.371 do Nucleation. 7A 0.154 47% HNOz, 0.037% Goodrun.

' Oaproie Acid. 1 7B 0.294 do No data could be obtamed. 8A 0.75447%HNO3, 1.3% Glu- Good data.

tarie and 0.7% Succinic. 8B 0.966 do Uaitplugged-Nucleaon. 9A 0.897 47%HINO1,5.2% Glu- Good data.

taric and 2.8% Succinic. 9B 1.080 ---..do Heavy frosting" (nucleation)on equip- I ment walls. 10 1.24 47% HNO, 7.7% Glu- Good run.

taric and 4.3% Succiuic.

The further processes of the examples which follow 1n Table II and inwhich parts are by weight unless otherwise stated, were carried out inthe manner described above. In Examples 1 through 8 the saturatedsolution from which the adipic acid was crystallized was preparedcontaining from 2 to 8 carbon atoms. Examples of the monobasic acids areacetic, and the normal and branched chain propionic, butyric, valeric,caproic, heptoic and caprylic acids. The dibasic acids that may be usedinby saturating a nitric acid mother liquor (NML) with clude, forexample, oxalic, malonic, succinic, glutaric, piadipic acid and inExample 9 through 12 by saturating a melic and suberic acids. watermother liquor (WML). The NML liquor was the The suppression ofnucleation can be eliected by the mother liquor left aftercrystallization of the first crop use of monobasic or dibasic acids orboth. If a mixture of adipic acid crystals from a product obtained fromthe is used the amounts required for optimum results are air and nitricacid oxidation of cyclohexane as described based on the relativeeflectivenes of the acids used as has in the Hamblet et al. U. S. Patent2,439,513, after stripbeen indicated above in the description of theamounts ping ofi the volatile matter. The WML was the mother of theseparate acids, the monobasic being considerably liquor left afterdissolving in water the first crop of adipic more efie tive than thedibasic acid. acid crystals referred to in the last sentence andcrystal- The process of the invention is more startingly etfecllZingadipic acid from that Water S01l1t1011- tive in large scalecrystallization than it is in the crystal- The value of K was determlned111 accord with the lizations described in the examples. In commercialuses equation: in which millions of pounds of adipic acid are crystaldwlized per year, an overall increase in capacity of as much K AS as 3,000pounds per hour has been realized when opde erating in accord with theinvention.

TABLE II Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 Ex. 7 Ex. 8 Ex. 9 Ex. 10Ex. 11 Ex. 12

Material used Weight charged, dry, gms- 4.00 5.00 3.00 3.00 3.00 5.00 5.00 5.00 3.00 3.00 3.00 3.00 iv. Growth rate, gms./hr 0.90 2.4 1.0 1.21.22 0.33 0.71 2.04 0.425 0.12 0.24 0.925

Bed height, dry, 051...; 2. 5 2. 5 1. 5 1. 5 1. 5 2. 5 2. 5 2. 5 1. 5 1.5 1. 5 1. 5 Bed volume, ems-.. 5.7 7.1 4.25 4.25 4. 7.1 7.1 7.1 4.254.25 4.25 4.25 Crystal volume cm 2.94 3.7 2.2 2.2 2.2 3.7 3.7 3.7 2.22.2 2.2 2.2 E 0.48 0.475 0.48 0. 48 0.48 0.48 0.48 0.48 0. 48 0.48 0.480.48 1, 450 1, 450 1, 450 1,450 1,450 1,450 1, 450 1, 450 1, 450 1, 4501, 450 1, 450 0.29 .30 0.218 0.218 0.218 0.3 0.30 0.30 0.218 0.218 0.2180. 21B

42 42 42 42 33 38 38 2. 2 2. 2 2. 2 2. 2 BA Glutaric and SucciniePercent DBA (adipic free) 12 8 8 8 8 8 8 8 (0.68 glutaric and 1.35succinic) MBA acetic valeric caproic all all all all all all all MBA,00:10., percent by wt 0 1. 25 0.10 0. 035 0. 245 0.245 0.245 0.0085 0.0085 0.0085 0. 0085 p; gm cc 1. 20 1. 20 1.20 1. 20 1.20 1.252 1.231.215 1.025 0.025 1.025 0.025 (In-'pf) gms/ce- 0.10 0.10 0.10 0.10 0.100.109 0.13 0.140 0.330 0.335 0.330 0. 3.30 low, InL/min. 95 9 95 95 9595 107 10 185 220 220 220 N0,ft./sec 0.0182 0. 0182 0. 0182 0. 0182 0.0182 0. 0182 0.0200 0.0192 0.0355 0.0423 0.0423 0.0423 Temperatures:

Av.'lernp.-Cryst. Bed, 0 38.34 40.10 37.72 37. 72 37.43 24.05 35.5940.04 40.12 27. 48 32. 50 40.80 Av. Temp.-Saturat0r, o 44.23 45.07 42.4842.84 43.21 31.70 30.20 48.74 40.30 29.73 34.50 42.14 supersaturation:

Supersaturation 0 5.89 4.97 4.70 5.12 5.78 7.11 3.07 2.10 0.24 2.25 2.001.28 supersaturation #/Fl;. solution 1.07 0.973 0.79 0.865 0.975 0.570.48 0.46 0.051 0.192 0.236 0.249 K, it /hr 0.0004 0.015 0.0128 0.0140.0120 0. 0035 0.0090 0.035 0.084 0.0003 0. 010 0. 0375 (1) =10-20 meshadipic acid from air and nitric acid oxidation of cycloher-zane.

where gg=growth rate (pounds or grams/hour) K =crystalliza'0ion lineargrowth rate constant (ft/hr) A=crystal surface area (ft?)S=supersaturation (lbs/ft. solution) A comparison of K in Examples 1through 5 in Table 11 illustrates that dibasic by-product acids decreasecrystallization rate as well as nucleation rate and that in the presenceof 8 to 12% dibasic acids other than adipic acid, the monobasic acids inless than 1.25% have no effect on rate. To compensate for the decreasedcrystallization rate imposed by the addition of by-product dibasic acidswhile maintaining their stabilizing effect, increased crystallizationtemperatures were used. A comparison of K in Examples 4 through 12 willshow the improved operation of a crystallization unit when operatedunder optimum temperature and supersaturation conditions. Between 6 and7 there is a 25-fold increase, between 6 and 8 a l0-fold improvement andbetween 10 and 12 a 6-fold improvement in the crystal growth rate.

The organic acids used to suppress nucleation of adipic acid during itscrystallization include those organic acids We claim:

1. In a process for crystallizing adipic acid the steps which comprisepreparing an aqueous solution containing adipic and nitric acids,saturating the solution, cooling said solution 2 C. to 5 C. below itssaturation temperature, crystallizing the adipic acid, suppressing therate of nucleation with at least 0.1% by weight of an organic inonobasicacid containing 2 to 8 carbon atoms, and increasing the rate of crystalgrowth by conducting the crystallization at temperatures between about40 C. and about 70 C.

2. The process of claim 1 conducted in the presence of at least 1% of anorganic dibasic acid other than adipic acid, containing 2 to 8 carbonatoms.

3. In a process for crystallizing adipic acid, the steps which comprisecooling an adipic acid-nitric acid containing aqueous solution, that issaturated with adipic acid, to 2 C. to 5 C. below its saturationtemperature, to eflfect supersaturation, crystallizing adipic acid fromthe supersaturated solution and suppressing the rate of nucleation by anorganic monobasic acid containing 2 to 8 carbon atoms present to theextent of at least 2% of the total nitric acid in solution, thecrystallization of the adipic acid from the aqueous nitric acidsupersaturated 7 solution being conducted at a temperature between about40 C. andabout 70 C. a

4. The process of claim 3 in which the rate of nuclea tion is alsosuppressed'by anorganic dibasic acid other than adipic acid, containing2 to 8 carbon atoms, present to the extent of at least 20% of the totalnitric acid in solution. 7

5. In a process for crystallizing adipic acid from a saturated crudereaction mixture thereof obtained by air oxidation followed by nitricacid. oxidation of cyclic hydrocarbons, the steps which comprise coolingthe saturated crude reaction mixture 2 C. to C. below its saturationtemperature, crystallizing the adipic acid, suppressing the rate ofnucleation by by-product organic monobasic acids present to the extentof at least 0.5% and increasing the rate of crystal growth by conductingthe crystallization at temperatures between about40 C. and about 70 C.

6. The process of claim 5 in which the nucleation is suppressed byby-product organic dibrasic acids present to the extent of at least 8%.

7. The process of claim 5 in which the nucleation is suppressed byrecycling a portion of the mother liquor from the crystallization zoneto the nitric acid oxidation to maintain a by-product monoand disbasicacid concentration between 8% and 18%.

8. In a circulatory process for the crystallization of adipic acid froma crude reaction mixture obtained by air and subsequent nitricacidoxidation of cyclic hydrocarbons in which by-product acids areformed, and wherein adipic acid supersaturation is continuously producedin a circulatory stream, the supersaturation being developed in one zoneand released by crystallization in another zone while at the same time amother liquor from the crystallization zone is returned to thesupersaturation zone after the addition of the crude reaction mixturefrom the air and nitric acid oxidation, the steps; which compriserecycling such a portion of the mother liquor as to maintain the nitricacid concentration, between 25 and and to increase the by-product acidconcentration to between 8 and 18%, supercooling such a solution to 2 C.to 5 C. below saturation temperature and crystallizing the adipic acidtherefromat a temperature between 40 and C. a a

9. in a ccntinuous process for the crystallization of adipic acid from acrude reaction mixture obtained by cyclohexane oxidation by air andsubsequent nitric acid oxidation of the resulting oxygenated product,wherein adipic acid is continuously crystallized from a 25-45% by Weightnitric acid solution, and the crude' adipic acid separated from themother liquor by filtration, the steps which comprise recycling aportion of the mother'liquor to the nitric acid oxidizers to maintainthe by-product dibasic acid concentration of 848% by weight, on anadipic acid free basis, in the crystallization environment, controllingthe rate of heat removal in the crystallization step to maintain atemperature between 40 and 70 C., and controlling the relativecrystallizer feed rate to crystallizer volume so as to maintain thecrystallization environment no more than 5 C. below its saturationtemperature.

OTHER REFERENCES Svanoe: J. Chem. Ed., pp. 549-53, October 1950.

1. IN A PROCESS FOR CRYSTALLIZING ADIPIC ACID THE STEPS WHICH COMPRISEPREPARING AN AQUEOUS SOLUTION CONTAINING ADIPIC AND NITRIC ACIDS,SATURATING THE SOLUTION, COOLING SAID SOLUTION 2*C. TO 5*C. BELOW ITSSATURATION TEMPERATURE, CRYSTALLIZING THE ADIPIC ACID, SUPPRESSING THERATE OF NUCLEATION WITH AT LEAST 0.1% BY WEIGHT OF AN ORGANIC MONOBASICACID CONTAINING 2 TO 8 CARBON ATOMS, AND INCREASING THE RATE OF CRYSTALGROWTH BY CONDUCTING THE CRYSTALLIZATING AT TEMPERATURES BETWEEN ABOUT40*C. AND ABOUT 70*C.